Allow relative paths in cpAsync again

[argosphil]

What does this PR do?

Fixes #9024 .

A recent commit in #8649 changed cpAsync to use

const fd = switch (Syscall.openatOSPath(bun.invalid_fd, src, open_flags, 0)) {

rather than

const fd = switch (Syscall.open(src, open_flags, 0)) {

The new code doesn't accept relative paths, which caused the issue.

This patch uses the CWD file descriptor instead.

• Code changes

How did you verify your code works?

Will add tests in a bit.

There is a test.

[gvilums]

I'm not quite sure that this is right - local testing shows that copying works just fine with relative paths. I think the issue is actually a combination of two separate issues:

• when copying to a directory that already exists, we throw an error
• the error we throw has a misleading error message

running

console.log(await fs.promises.cp("abc", "def", { recursive: true }));

(assuming abc exists and contains foo.txt, and def doesn't exist) works, but if def exists it fails with

EBADF: Bad file descriptor
   errno: -9
 syscall: "open"
   path: "abc"

This seems to be the error we're seeing. I don't think this is related to relative path processing, but rather the fact that we don't correctly handle the case where the directory already exists.

Node transparently does the copy if the directory already exists.

[gvilums]

Disregard the above, I mixed up which use of openatOSPath you meant 😅

[argosphil]

Disregard the above, I mixed up which use of openatOSPath you meant 😅

Thank you, I was confused myself and really shouldn't have opened this without a test. There is one now. Criticism and nitpicking appreciated :-)

[Jarred-Sumner]

Thank you